Get the Gerrit users emails as a string list using ssh and gsql query.

gerrit_users.py

class GerritUsersListProvider(object):
  def get_users(self):
    """Gets the Gerrit users emails as a string list."""
    server = os.environ.get('GERRIT_SERVER', GERRIT_SERVER)
    logging.debug('Querying all user accounts from ' + server)
    text = subprocess.check_output([
      'ssh', '-p', '29418', server, 'gerrit', 'gsql', '--format',
      'JSON_SINGLE', '-c', "'select preferred_email from accounts'"])
    accounts_json = json.loads(text)
    return self.json_to_list(accounts_json)

  def json_to_list(self, accounts_json):
    """Converts the JSON returned by Gerrit to a simple array of emails."""
    users = [row['columns']['preferred_email']
             for row in accounts_json
             if 'columns' in row and 'preferred_email' in row['columns']]
    return users